Electrical series circuits such as a series circuit of low voltage light bulbs are known. Low voltage loads such as light bulbs may be connected in series to built up to a normal line voltage. This is the usual "Christmas Tree" light string scheme. However, if one bulb burns out, the whole string will extinguish. For Christmas tree lights, this is not a big problem, because the strings of lights are inexpensive and can be easily discarded and replaced. There are some types of these string lights which are intended to continue to light even if some bulbs are extinguished. They make use of a spring as one of the terminals connecting to the bulb filament. The spring keeps the filament in tension. When the filament burns out, the spring travels and makes contact with the other terminal for the filament, thus shorting that bulb out. No provision is made for keeping the voltage across the remaining bulbs from increasing. This increase of voltage could cause premature burnout of other bulbs. As at least a partial offset it might be possible to start with a minimum operating voltage so that the burnout of a small number of bulbs can be tolerated.
In the 1920's and 30's municipalities employed series circuits for street lights in order to save on the cost of copper. To avoid the opening of the whole series circuit if one of the lamps should burn out, the lamps in a high voltage series circuit are provided with by-pass protectors connected in parallel with them. Current regulators were employed to provide constant current even if one light was burned out. If one of the lamps should burn out, the full voltage of the series circuit is then impressed on the by-pass protector. The excessive voltage breaks down the insulating film in the by-pass protector which then becomes conducting and permits the remainder of the circuit to continue in operation.
In other lighting applications, the by-pass protector was a thin disc of paper held between a pair of contacts in parallel with a light bulb for each bulb connected in a series circuit. The thin paper acted as insulation between the contacts and current flowed through each light bulb to light the bulb. Only about a hundred Volts appeared across each bulb which was not enough to burn through the thin paper. However, if a bulb burned out, 5-10 K Volts would appear across the contacts and burn out the thin paper so that the contacts would make a connection. Consequently, current would flow in a path parallel to the burned out light bulb and the remaining bulbs would light. A current regulator would ensure that the remaining bulbs would operate at their previous voltage.
The above electric series circuits do not provide an adequate solution to the problem of an entire string of series connected loads such as light bulbs becoming extinguished when one load burns out for most applications. In many lighting applications, it is not possible to bear the cost of discarding the string of series connected bulbs. It is undesirable for the life of the remaining bulbs to be significantly decreased. Further current regulators are expensive and bulky for low voltage lighting applications. Additionally, insulating film by-pass protectors are not appropriate for low voltage lighting. Further, inserting discs of thin paper in electric circuits is not practical nor effective enough to meet modem standards for low voltage electric lighting.
There is, therefore, a need for an electric series circuit for low voltage loads such as light bulbs in which current flows through the plural light bulb loads connected in series even if one or more of the loads bums out, in order to identify the burned out load or loads for replacement. Further, there is a need for an electric series circuit which is practical for use in modem lighting applications. There is a need for an electric series circuit that permits current to flow through the light bulb loads that are not burned out and which does not significantly reduce the life of the remaining bulbs. Additionally, there is a need for an electric series circuit which is simple in construction and inexpensive and which permits current to flow through a string of series connected loads such as light bulbs when one or more loads have burned out. There is a need for an electric series circuit for assuring current flow to a string of series connected loads such as light bulbs when one bulb burns out which does not require a current regulator. There is a need for an electric series circuit that permits current to flow to a string of series connected loads when one load stops conducting current for low voltage loads. There is a need for a transformerless low voltage electric series circuit.